High temperature stable yellow and yellow-orange pigments are highly valued. Pigments with high chromaticity and temperature stability possessing required color characteristics have traditionally been made from cadmium sulfide, cadmium sulfo-selenide or zinc substituted cadmium sulfo-selenides.
Because of regulatory concerns the use of these cadmium containing pigments described above has been increasingly constrained in commerce. Thus, there is a need for substitutes that meet the color characteristics of CdS or CdS—Se pigments without depending on cadmium.
Tin niobium pyrochlore is formally Sn2Nb2O7. The use of this general class of compound as a pigment or colorant to replace cadmium sulfide or sulfo-selenide pigments is proposed and disclosed in U.S. Pat. No. 7,594,961.
The disclosure in the above referenced patent contemplates making the pure pyrochlore, Sn2Nb2O7. Additionally, the referenced application posits pigments in which tin sulfide is used in lieu of stannous oxide in the synthesis of the finished pigment. Finally, the referenced patent posits substitution of the pentavalent niobium atoms with other transition metals that can exhibit a high valence state, such as tungsten or molybdenum in the form of MoO3 or WO3. These substitutions are intended to modify the color of the pigments produced.
All syntheses described in the above referenced patent were either conducted in a sealed tube, or performed under controlled atmosphere conditions. Despite such care, acid-washing step of the initial product is required to yield a clean, vibrant pigment. This acid washing removes either unreacted stannous oxide or metallic tin.